The process of preparing optical or ophthalmic lenses begins usually with an unfinished or semi-finished glass or plastic lens blank. Typically a semi-finished lens blank has a finished polished front surface and an unfinished back surface. By grinding away material from the back surface of the lens blank the required corrective prescription is generated. Thereafter the surface, having had the corrective prescription imparted thereto, is polished and the peripheral edge of the thus processed lens blank is provided with a final desired contour thereby establishing a finished optical or ophthalmic lens. According to other processing methods, the finished ophthalmic lens can be directly processed from a lens blank using for example three directional machining. The lens blank can be either a plastic or a glass lens blank.
It is necessary during these various processing operations to securely maintain the lens blank in accurate alignment and in place on the lens blocking device. This procedure is often referred to as “lens blocking”.
Heretofore, various materials were employed to secure the lens blank to the lens blocking device. These materials include glues, pitch and low temperature fusible metal alloys. The use of glues and pitch, in addition to being messy, suffers the further disadvantage of generally being non-reusable or non-reclaimable. While the prior art use of low temperature metal alloys eliminated some of these disadvantages experienced with the use of glues and pitch, nonetheless, the use of these metal alloys, both in their preparation and their reclamation caused significant environmental and health hazards especially since these alloys were most often fabricated from such metals as cadmium, tin, lead and bismuth. Of these metals, lead and cadmium are the most toxic. Lead is strong protoplasmic poison and can be introduced into the body by ingestion, inhalation and skin absorption. Cadmium poisoning is similar to lead in many ways and is introduced into the body in the same way as lead. Like lead, cadmium is stored in the liver, kidney and bone. Procedures for the formulation of such alloys and reclamation processes so as to enable its re-use as a material to secure a lens blank to a lens block thus exposes workers to serious environmental and health hazards. The formation of these alloys often is accomplished through the use of powdered or particulate metals which are subjected to a sintering and heat treating process. Fumes and/or dust particles of these metals are released to the ambient atmosphere thereby creating environmental and health hazards for those formulating these alloys. The same hazards exist for those attempting to reclaim the used low temperature metal alloy blocking material.
To overcome these issues, organic low shrinkage materials have been developed to be used as lens blocking materials.
U.S. Pat. No. 6,036,313 in the name of 3M Innovative Properties Company discloses examples of compound families suitable for lens blocking with thermoplastic materials.
The disclosed blocking compositions have many advantages over traditional metal alloy materials. For example, the lens blocking compositions are non-toxic, environmentally safe, and preferably biodegradable. The materials preferably can be used with existing processing equipment and may be recycled. An optical lens blocking device can be used that comprises a solidified mass of a thermoplastic blocking composition. The blocking composition is solid at 21° C. and has a low melting point.
There remains a need for improving blocking material for blocking a lens blank to a lens blocking device thanks a cheap, non-toxic and easy to handle material.